1. Field of the Invention
The present invention relates to the development of an active material useful for an anode of lithium ion secondary cells.
2. Description of Prior Arts
Most commercially available rechargeable lithium ion batteries adopt highly ordered carbonaceous materials as active materials for their anodes. Generally, highly ordered carbonaceous materials are those which have a crystal lattice Lc(002).gtoreq.50 .ANG. with a distance between carbon layer planes d.sub.002.ltoreq.3.4 .ANG.. Showing a Coulomb effect with a low flat potential curve upon discharging in addition to being low in moisture content and relative impurity content, they are known to be easily applicable in practical processes. Where cost is an important determinant in commercializing the highly ordered carbonaceous materials, they are very disadvantageous in that they are decomposed by reaction to electrolytes and need a high temperature treatment and a high purification treatment. What is worse, they have a theoretical discharge capacity as low as 372 mAhg.sup.-1. Therefore, there remains a need to develop novel carbonaceous materials which have a larger discharge capacity for high capacity rechargeable batteries.
Recently, there have been reported many research results on carbonaceous materials of low crystallinity which have a higher capacity than the theoretical charge and discharge capacity of the highly ordered carbonaceous materials. Particularly, hard carbons, although suffering from a low initial Coulomb effect and a difficult charging process, attract scientific attention by virtue of their higher electric capacity (400.sup..about. 650 mAhg.sup.-1) than the theoretical value of graphite, low reactivity with electrolytes, and low production cost according to relatively low temperature treatment.
Yoshino et al., prepared a carbonaceous material for lithium ion secondary cells by the pyrolysis of benzene gas. Dahan et al., reported that a disordered carbonaceous material with a high capacity could be prepared by doping boron (B) through a chemical vapor deposition (CVD) process. As for carbonaceous materials prepared by vapor deposition, thus far, most of them are of pyrolyzed carbons deposited on substrates. In fact, because only a very small quantity of carbonaceous materials are deposited through thermal CVD, they are very disadvantageous in productivity and production cost when using them as active materials for the anodes of lithium ion batteries.
There are reports regarding the preparation of carbonaceous materials for anodes of lithium ion rechargeable batteries from solid phase carbon-containing precursors through thermal treatment. For example, polyparaphenylene (Sato et al.), coal-tar pitch (Mabuchi et al.), a phenolic resin (Yara), and sugar carbon (Xing and Xue) are subjected to thermal treatment to produce disordered carbonaceous materials of high charge and discharge capacity. Also, it is reported that various solid phase precursors, such as petroleum pitch, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyphenylene sulfide (PPS), and epoxy novolac resin (ENR), can be thermally treated to produce disordered carbonaceous materials which show largely two charging and discharging properties depending on the precursors. Particularly, when the carbonaceous materials prepared are of hard carbon, they are reported to be more suitable as active materials for anodes of lithium ion rechargeable batteries. However, since the solid thermal treatment process for the preparation of carbonaceous materials has fewer controllable parameters than have vapor deposition processes, it suffers from disadvantages in that it is inconvenient to conduct the structure control of carbonaceous materials, and it is very difficult to add other elements and to control their amounts.